Electromechanical generator for converting mechanical vibrational energy with magnets and end cores into electrical energy

ABSTRACT

An electromechanical generator for converting mechanical vibrational energy into electrical energy, the electromechanical generator comprising: a central mast, an electrically conductive coil assembly fixedly mounted to the mast, a mount for the coil assembly, a magnetic core assembly movably mounted to the mast for vibrational motion along an axis, wherein the magnetic core assembly comprises: an outer core, comprising a one-piece tubular body, which encloses the electrically conductive coil assembly side, first and second end cores magnetically coupled to the outer core at respective first and second ends of the outer core, the first and second end cores extending radially inwardly and enclosing respective first and second opposite edges of the coil assembly, and first and second magnets spaced along the axis, contacting and being magnetically coupled to the respective first and second end cores, and defining therebetween a gap in the magnetic core assembly through which the mount extends.

CROSS REFERENCE TO RELATED APPLICATION

This application is a US 371 application from PCT/EP2019/057221 entitled“An Electromechanical Generator for Converting Mechanical VibrationalEnergy into Electrical Energy” filed on Mar. 22, 2019 and published asWO 2019/185472 A1 on Oct. 3, 2019, which claims priority to GBApplication 1804871.0 filed on Mar. 27, 2018. The technical disclosuresof every application and publication listed in this paragraph are herebyincorporated herein by reference.

BACKGROUND TO THE INVENTION

The present invention relates to an electromechanical generator forconverting mechanical vibrational energy into electrical energy, i.e. avibration energy harvester. In particular, the present invention relatesto such a vibration energy harvester which is a miniature generatorcapable of converting ambient vibration energy into electrical energyfor use, for example, in powering intelligent sensor systems. Such avibration energy harvester can be used in many areas where there is aneconomical or operational advantage in the elimination of power cablesor batteries.

DESCRIPTION OF THE PRIOR ART

It is known to use vibration energy harvesters comprising anelectromechanical generator for harvesting useful electrical power fromambient vibrations, e.g. for powering wireless sensors. A typicalmagnet-coil generator consists of a spring-mass combination attached toa magnet or coil in such a manner that when the system vibrates, a coilcuts through the flux formed by a magnetic core.

The Applicant's earlier U.S. Pat. Nos. 7,586,220 and 8,492,937 disclosevibration energy harvesters in the form of an electromechanicalgenerator in which a ferromagnetic core, connected to two magnets, eachlocated at a respective end of the magnetic core, is provided. Theassembly of the ferromagnetic core and the two magnets has asubstantially C-shaped cross section. The magnetic core assemblysurrounds and encloses a coil. The magnetic core assembly isspring-mounted to a central tubular body or mast and the magnetic coreassembly can be linearly vibrated in a direction along the axis of themast to cause vibrational motion of the magnetic core assembly relativeto the coil to produce electrical energy in the coil.

Each of U.S. Pat. Nos. 7,586,220 and 8,492,937 discloses that the coreis manufactured in two parts, the two parts being substantially“cup-shaped”. To assemble the magnetic core assembly, one cup-shapedpart is inverted and placed over the other cup-shaped part and a centraljoint is provided where the two “rims” of the cup-shaped parts arejoined together to form a unitary core. In these prior patentspecifications, each “rim” has an upstanding flange and an adjacentrecess, and in the resultant assembly each flange is received in arecess of the other part. This provides a secure central joint in themagnetic core assembly. The magnetic core assembly is substantiallysymmetric about the axis. Such a structure can minimise the number ofjoints required to form the magnetic core assembly and can preserve asmuch symmetry as possible within the core structure. Furthermore,interference with the magnetic field is also minimised if the singlejoint is centrally located along the axis.

However, it has been found that in order to achieve high electricalpower output from mechanical vibrational energy in a pre-specified inputfrequency band, it is necessary for the device to be structuredaccurately to control of the motion of the magnetic core assembly in theenergy harvester, and that to achieve such accurate control the axiallength of the magnetic core assembly must be defined accurately. Theknown magnetic core assembly structure described above requires twoprecise machining operations. Also, the manufacture of a single centraljoint described above requires a precise machining operation of theflange and recess.

There is a need in the art to provide a magnetic core assembly which canprovide a highly accurate axial length of the magnetic core assembly yetwhich can be readily manufactured with low manufacturing complexity andcost, in particular by minimizing or reducing the number of precisionmachining operations required to manufacture the ferromagneticcomponents of the magnetic core.

SUMMARY OF THE INVENTION

The present invention aims at least partially to provide an energyharvester in the form of electromechanical generator which can provide amagnetic core assembly which can reliably provide a highly accurateaxial length of the magnetic core assembly yet which can be readilymanufactured with low manufacturing complexity and cost, in particularby minimizing or reducing the number of precision machining operationsrequired to manufacture the ferromagnetic components of the magneticcore.

The present invention accordingly provides an electromechanicalgenerator for converting mechanical vibrational energy into electricalenergy, the electromechanical generator comprising: a central mast, anelectrically conductive coil assembly fixedly mounted to the mast, thecoil assembly at least partly surrounding the mast, the coil assemblyhaving radially inner and outer sides and first and second oppositeedges, a mount for the coil assembly extending radially inwardly of theradially inner side and fixing the coil assembly to the mast, a magneticcore assembly movably mounted to the mast for linear vibrational motionalong an axis about an equilibrium position on the axis, the magneticcore assembly at least partly surrounding the coil assembly and themast, wherein the magnetic core assembly comprises: an outer core,comprising a one-piece tubular body, which encloses the electricallyconductive coil assembly on the radially outer side, first and secondend cores magnetically coupled to the outer core at respective first andsecond ends of the outer core, the first and second end cores extendingradially inwardly and enclosing the respective first and second oppositeedges of the coil assembly, wherein either (i) both of the first andsecond end cores are fitted to and contact the outer core at therespective first and second ends of the outer core, or (ii) one of thefirst and second end cores is fitted to and contacts the outer core atthe respective first or second end of the outer core and the other ofthe first and second end cores is integral with the outer core at therespective first or second end of the outer core, and first and secondmagnets spaced along the axis, wherein the first and second magnetscontact and are magnetically coupled to the respective first and secondend cores, and the first and second magnets define therebetween a gap inthe magnetic core assembly through which the mount extends.

Preferred features are defined in the dependent claims.

The present invention is predicated on the finding that a magnetic corecan be manufactured by joining a substantially tubular outer componentto one or two substantially flat inner components, each inner componentbeing located at a respective end of the core. The substantially tubularouter component can define a single axial length of the core, thetolerance of which can be readily defined by a precision grindingoperation. The magnetic core can be manufactured with low manufacturingcomplexity and low cost.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way ofexample only with reference to the accompanying drawings, in which:

FIG. 1 is a schematic side section through an electromechanicalgenerator for converting mechanical vibrational energy into electricalenergy in accordance with a first embodiment of the present invention;and

FIG. 2 is a schematic plan view of a spring in the electromechanicalgenerator of FIG. 1 ;

FIG. 3 is a schematic plan view of an end core part in anelectromechanical generator in accordance with a second embodiment ofthe present invention; and

FIG. 4 is a schematic side section through an electromechanicalgenerator for converting mechanical vibrational energy into electricalenergy in accordance with a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The electromechanical generator of the present invention is a resonantgenerator known in the art as “velocity-damped” where substantially allof the work done by the movement of the inertial mass relative to thehousing is proportional to the instantaneous velocity of that movement.Inevitably, a portion of that work is absorbed overcoming unwantedmechanical or electrical losses, but the remainder of the work may beused to generate an electrical current via a suitable transductionmechanism, such as the electrical coil/magnetic assembly describedbelow.

FIGS. 1 and 2 illustrate an electromechanical generator 2 for convertingmechanical vibrational energy into electrical energy in accordance witha first embodiment of the present invention. In operation, theelectromechanical generator 2 is enclosed within a housing (not shown)and the device is provided with a fitting (not shown) for securelymounting the electromechanical generator 2 to a support (not shown) fromwhich support mechanical vibrational energy is harvested which isconverted into electrical energy by the electromechanical generator 2.

The electromechanical generator 2 comprises a central mast 4 extendingalong longitudinal axis A-A. In use, the amplitude of the inputmechanical vibrational energy is typically along, or has a componentextending along, the longitudinal axis A-A. The opposite ends 6, 8 ofthe mast 4 are fitted to the housing (not shown) and one or both ends 6,8 of the mast 4 may be provided with a fitting (not shown), for examplea threaded hole, for securely mounting the electromechanical generator 2to a support, or to a housing.

Preferably the mast 4 is made from a low-permeability, low-conductivity,but high-elastic-modulus material, such as 316 stainless steel. The mast4 may be at least partly hollow, with a central hollow bore 5.

An electrically conductive coil assembly 10 is fixedly mounted to themast 4. The coil assembly 10 at least partly, preferably wholly,surrounds the mast 4. The assembly 10 comprises an electricallyconductive coil 12 which is circular and is coaxial with the mast 4. Theassembly 10 has radially inner and outer sides 14, 16, the sides 14, 16extending parallel to the axis of rotation A-A. The assembly 10 hasfirst and second (typically upper and lower) opposite edges 18, 20. Thecoil 12 has first and second coil portions 13 a, 13 b respectivelylocated adjacent to the first and second opposite edges 18, 20.

A mount 22 for the coil assembly 10 extends radially inwardly of thecoil assembly 10 and fixes the coil assembly 10 to the mast 4. The mount22 extends radially inwardly of the radially inner side 14. The coil 12is mounted within an annular coil support 24 of the mount 22. In thisspecification the term “annular” means “ring-like” but does not implyany other specific geometric shape and does not imply that the plan viewof any annular element must be rounded; for example the sides of the“annular” or “ring-like” element may be straight. In the illustratedembodiment, the annular coil support 24 is preferably circular in plan,but may be any other ring-like geometric shape. Similarly, the otherannular elements described herein are also preferably circular in plan,but may be any other ring-like geometric shape. This assembly 10 mountsthe coil 12 in a fixed position within the housing. The coil support 24is located outwardly in a radial direction from the axis A-A and alsosubstantially midway in an axial direction between the ends 6, 8 of themast 4.

The mount 22 comprises a conical wall 26 extending between the coilassembly 10 and the mast 4. The conical wall 26 is integral with theannular coil support 24. The annular coil support 24 includes a radiallyoriented inner wall 28 which connects to the conical wall 26. Theconical wall 26 is a moulded body, preferably injection moulded,composed of a thermoplastic material, and the moulded body comprises theannular coil support 24 and the conical wall 26. Preferably thethermoplastic material is a very low-conductivity material, such asglass-loaded plastic.

The conical wall 26 has opposite first and second ends 30, 32. The firstend 30 has a smaller diameter than the second end 32. The first end 30is mounted to the mast 4 and the second end 32 is mounted to the coilassembly 10. The mount 22 further comprises an inner wall 34 integralwith the first end 30. The inner wall 34 is arcuate, or curved, andfitted around at least a portion of the circumference of a middleportion 36 of the mast 4.

In the illustrated embodiment, the conical wall is inclined at an angleof from 40 to 50 degrees to the axis A-A, typically at an angle of about45 degrees to the axis A-A. Preferably, a central part 38 of the conicalwall 26 is located substantially midway in an axial direction along themast 4.

The mount 22 for the coil assembly 10 preferably defines a recess (notshown) in which is received circuitry (not shown) for electricallyconditioning the electrical output of the coil 12, for example byvoltage regulation. The circuitry is preferably encapsulated by aplastic or rubber sealing material, which seals and protects thecircuitry against undesired environmental influences, such as humidity,liquids, etc. The coil 12 is connected the circuitry by wires (notshown) and in turn the circuitry has second wires (not shown) extendingtherefrom for connecting to external circuitry (not shown).

A magnetic core assembly 40 is movably mounted to the mast 4 for linearvibrational motion along the axis A-A about an equilibrium position onthe axis A-A, the equilibrium position being illustrated in FIG. 1 . Themagnetic core assembly 40 at east partly, preferably wholly, surroundsthe coil assembly 10 and the mast 4.

The magnetic core assembly 40 comprises two opposed magnetic circuitsspaced along the axis A-A. In the illustrated embodiment, the magneticcore assembly 40 comprises a pair of axially aligned annular first andsecond magnets 42, 44 spaced along the axis A-A. The magnets 42, 44, areeach typically a rare earth permanent magnet having a high magneticfield strength. Poles of the magnets 42, 44 having a first commonpolarity face towards each other, and poles of the magnets 42, 44 facingaway from each other are of a second common polarity.

A ferromagnetic body 46 contacts and is magnetically coupled to thepoles of the magnets 42, 44 facing away from each other. Generally, theferromagnetic body 46 extends radially outwardly of the magnets 42, 44relative to the axis. The ferromagnetic body is generally tubular andhas radially inwardly extending arms at each end thereof, each armmounting a respective magnet 42, 44 thereon.

The magnets 42, 44 are mounted on opposite sides of, in FIG. 1 above andbelow, the conical wall 26 and radially inwardly of the coil 12. Themagnets 42, 44 are each axially spaced from the conical wall 26, anddefine a gap 48 through which the mount 22, in particular conical wall26, extends. The magnets 42, 44 are aligned so that their identicalpoles face each other on opposite sides of the conical wall 26.

The magnetic core assembly 40 comprises an outer core 50, comprising aone-piece tubular body 52, which encloses the electrically conductivecoil assembly 10 on the radially outer side 16. The tubular body 52 iscylindrical.

The magnetic core assembly 40 further comprises first and second endcores 54, 56 contacting and magnetically coupled to the outer core 50 atrespective opposite first and second ends 58, 60 of the outer core 50.The first and second end cores 54, 56 extend radially inwardly andenclose the respective first and second opposite edges 18, 20 of thecoil assembly 10. The magnetic core assembly 40 further comprises thefirst and second magnets 42, 44 spaced along the axis A-A. The first andsecond magnets 42, 44 contact and are magnetically coupled to therespective first and second end cores 54, 56. The first and second coilportions 13 a, 13 b are respectively at least partly located between theouter core 50 of the common ferromagnetic body and one of the magnets42, 44.

The first and second ends 58, 60 of the tubular body 52 each comprise arecess 62, 64 on an inner side 66 of the tubular body 52. The first andsecond end cores 54, 56 are fitted in the recess 62, 64 of therespective first and second ends 58, 60 of the tubular body 52.

The recess 62, 64 has a transverse mounting surface 68 facing along theaxis A-A away from the equilibrium position and a longitudinal mountingsurface 70 facing towards the axis A-A. Radial and circumferentialsurfaces 72, 74 of the respective first and second end cores 54, 56 arerespectively fitted to the transverse and longitudinal mounting surfaces68, 70.

The first and second end cores 54, 56 comprise plates. The first andsecond end cores 54, 56 may be planar or may be provided with somethree-dimensional shaping on the outer or inner surfaces. The first andsecond end cores 54, 56 are circular, each having an outercircumferential surface 74 fitted to an inner circumferential surface,which is the longitudinal mounting surface 70, of the outer core 50 anda central hole 76 surrounding the mast 4. In the illustrated embodimentthe first and second end cores 54, 56 are circular discs which arefitted into the ends of the tubular body 52. The circular circumferenceof the first and second end cores 54, 56 may be axially fitted toshoulders, formed by the transverse and longitudinal mounting surfaces68, 70, on the inner side 66 of the tubular body 52. The fitting may bea pressure, relaxation or elastic fit. The first and second end cores54, 56 may be optionally bonded to the tubular body 52. The resultantstructure provides a substantially C-shaped magnetic core withsubstantially uniform ferromagnetic properties, and an accurate axiallength.

In a modified embodiment, as shown in FIG. 3 , the first and second endcores 54, 56 are circular and have a small angular segment cut-out oropening 55 which enables the first and second end cores 54, 56 to bepress-fitted into the recess 62, 64 of the respective first and secondends 58, 60 of the tubular body 52. The first and second end cores 54,56 are composed of a compliant material and are oversize relative to theinternal dimensions of the recess 62, 64, and the elastic relaxation ofthe first and second end cores 54, 56 in the recess 62, 64 ensures axialretention of the first and second end cores 54, 56 in the tubular body52 so that the magnetic core assembly has an accurate axial length.Since the magnetic field in the first and second end cores 54, 56 isradial, and a cut-out or opening 55 which is substantially radial onlyminimally affects the magnetic circuit. The installed first and secondend cores 54, 56 exert outward pressure on the tubular body 52, whichcompletes the required magnetic circuit efficiently.

In a further embodiment, as shown in FIG. 4 , one (rather than both asshown in FIG. 1 ) of the first and second end cores, in the illustratedembodiment the second end core 56, is integral with the tubular body 52,and the first end core 54 has the structure described above withreference to the embodiment of FIG. 1 . At the opposite end, the tubularbody 52 is provided with an integral end core part 57. The non-integralend core 54 may have either construction as described above, inparticular either continuous or with a cut-out or opening 55 which issubstantially radial.

First and second locator elements 78, 80 are respectively fitted to thefirst and second end cores 54, 56. The first and second locator elements78, 80 each extend towards the mount 22. Each of the first and secondlocator elements 78, 80 has a locating surface 82 which engages a sidesurface of a respective first and second magnet 42, 44. The first andsecond locator elements 78, 80 are fitted to a fitting surface 84, 86 ofthe respective first and second end cores 54, 56, the fitting surface84, 86 facing towards the axis A-A. The locating surface 82 of the firstand second locator elements 78, 80 is fitted to a side surface 88, 90 ofthe respective first and second magnets 42, 44, the side surface 88, 90facing towards the axis A-A. The first and second locator elements 78,80 accurately and securely fit the magnets at the correct location inthe magnetic core assembly 40.

The magnetic core assembly 40, comprising the radially outer core 50,first and second end cores 54, 56 and radially inner magnets 42, 44,defines therebetween an annular enclosed cavity 92 in which the coil 12is received. The magnets 42, 44 and the outer core 50 and first andsecond end cores 54, 56 are slightly spaced from the coil 12 to permitrelative translational movement therebetween. The magnetic core assembly40 has a substantially C-shaped cross-section and is rotationallysymmetric.

The cavity 92 has respective cavity portions between each of the firstand second coil portions 13 a, 13 b and the central mast 4, and above orbelow, respectively, the conical wall 26 of the mount 22.

The outer core 50 and first and second end cores 54, 56 are composed ofa ferromagnetic material having a high magnetic permeability, and a highmass, such as soft iron. The assembly of the outer core 50, first andsecond end cores 54, 56 and the magnets 42, 44 therefore forms twoaxially spaced magnetic circuits of the magnetic core assembly 40. Thelimits of the lines of magnetic flux of each magnetic circuit aredefined by the outer core 50 and the respective first and second endcores 54, 56, which substantially prevents magnetic flux from eachmagnet 42, 44 extending axially or radially outwardly from the commonferromagnetic body formed of the outer core 50 and first and second endcores 54, 56. Since the opposed magnets 42, 44 face each other withcommon poles, at the central region of the magnetic core assembly 40 themagnetic flux of the opposed magnetic circuits are in opposition therebydirecting the magnetic flux radially outwardly towards the commonferromagnetic body.

The resultant effect is that a single magnetic core assembly 40comprises two separate magnets 42, 44 and each has a respective magneticcircuit in which a very high proportion of the magnetic flux isconstrained to pass through the respective coil portion 13 a, 13 b. Thisin turn provides a very high degree of magnetic coupling between themagnets 42, 44 and the coil 12. Consequently, any relative movementbetween the magnets 42, 44 and the coil 12, in particular as describedbelow by linear axial resonant movement of the magnetic core assembly 40relative to the fixed coil 12, produces a very high electrical poweroutput at the coil 12.

A biasing device 100 is mounted between the mast 4 and the magnetic coreassembly 40 to bias the magnetic core assembly 40 in opposed directionsalong the axis A-A towards the equilibrium position. The biasing device100 comprises a pair of first and second plate springs 102, 104. Each ofthe first and second plate springs 102, 104 has an inner edge 106, 108respectively fitted to the first and second opposite ends 6, 8 of themast 4 and an outer edge 114, 116 fitted to the magnetic core assembly40. The outer edge 114 of the first plate spring 102 is fitted to afirst end part 118 of the magnetic core assembly 40 and the outer edge116 of the second plate spring 104 is fitted to a second end part 120 ofthe magnetic core assembly 40.

Each of the first and second plate springs 102, 104 comprises a springmember 122, 124 comprising an inner portion 126, 128, which issubstantially orthogonal to the axis A-A and includes the respectiveinner edge 106, 108, and a cylindrical outer portion 130, 132 which issubstantially parallel to the axis A-A and includes the respective outeredge 114, 16.

The spring member 122, 124 is a folded sheet spring and the inner andouter portions 126, 128; 130, 132 are connected by a fold 134, 136.

Each outer edge 114, 116 is fitted to an outer circumferential surface138, 140 of the magnetic core assembly 40. In the illustratedembodiment, each outer edge 114, 116 is push-fitted onto the outercircumferential surface 138, 140 of the magnetic core assembly 40 andfitted thereto by an elastic fit.

The inner edge 106, 108 of each of the first and second plate springs102, 104 is fitted to the mast 4 by a riveted joint 142, 144 between theinner edge 106, 108 and the mast 4.

The first and second plate springs 102, 104 each apply the samemechanical biasing force against the magnetic core assembly 40 when themagnetic core assembly 40 is moved away from the central equilibriumposition. The first and second plate springs 102, 104 preferably havethe same spring constant.

The provision of a pair of first and second plate springs 102, 104 atopposed axial ends of the movable magnetic core assembly 40 provides astructure that can not only provide a sufficient spring biased restoringforce on the magnetic core assembly 40 to bias it towards an axiallycentral position with respect to the coil 12, but also takes upsubstantially minimum volume within the housing. In particular, thelocation of the first and second plate springs 102, 104 at opposed axialends of the movable magnetic core assembly 40 can help to maximize thesize of the magnetic core assembly 40 for a given device volume whichnot only maximizes the magnetic coupling, but also importantly permitsthe mass of the movable magnetic core assembly to be correspondinglymaximized. As known in the art, there is a desire to maximize the massof the movable magnetic core assembly in a resonant vibrationalelectromagnetic energy harvester because this increases the outputelectrical power.

The provision of a pair of first and second plate springs 102, 104 alsoavoids the need for expensive and cumbersome helical springs surroundingthe movable magnetic core assembly. This decreases the manufacturingcost by reducing the component cost.

Referring now to FIG. 2 which shows a plan view of the first and secondplate springs 102, 104, the inner portion 126, 128 is substantiallycircular. In the illustrated embodiment, each inner portion 126, 128 hasan outer circumferential part 146 adjacent to the fold 134, 136, aninner circumferential part 148 adjacent to the inner edge 106, 108, andat least three arms 150, 152, 154 connecting together the outer andinner circumferential parts 146, 148. The arms 150, 152, 154 aremutually spaced around the axis A-A and each pair of adjacent arms 150,152, 154 is separated by a respective opening 156, 158, 160therebetween. The arms 150, 152, 154 are equally mutually spaced aroundthe axis A-A. With three arms 150, 152, 154, the angular separationbetween the same parts of adjacent arms 150, 152, 154 is 120 degrees.

Each arm 150, 152, 154 comprises a radial outer part 162 connected tothe outer circumferential part 146, a middle circumferential part 164connected to the radial outer part 162, and a radial inner part 166connected between the middle circumferential part 164 and the innercircumferential part 148.

Each opening 156, 158, 160 comprises an outer circumferential region 168adjacent to the outer circumferential part 146, a middle radial region170 connected to the outer circumferential region 168, and an innercircumferential region 172 connected to the middle radial region 170 andadjacent to the inner circumferential part 148.

Each opening 156, 158, 160 extends between outer and inner opening ends174, 176, each of the opening ends 174, 176 having an enlarged width ascompared to the adjacent portion of the opening 156, 158, 160. Thisreduces stress concentrations in the spring.

In the illustrated embodiment, there are exactly three arms 150, 152,154 and exactly three openings 156, 158, 160. This provides an angularseparation of 120 degrees between the arms, and between the openings. Inother embodiments, more than three arms and correspondingly more thanthree openings, are provided. For example, four, five or sixarms/openings may be provided, with respective angular separations of90, 72 and 60 degrees between the arms, and between the openings.

Preferably, as in the illustrated embodiment, the arms 150, 152, 154have the same shape and dimensions and correspondingly the openings 156,158, 160 have the same shape and dimensions.

The electromechanical generator 2 further comprises a pair of first andsecond spacers 178, 180. The first spacer 178 is fitted between thefirst plate spring 102 and a first surface 182 of the mast 4, and thesecond spacer 180 is fitted between the second plate spring 104 and asecond surface 184 of the mast 4. The first and second surfaces 182, 184are located at the respective first and second opposite ends 6, 8 of themast 4.

A resilient device 186 is mounted between the biasing device 100 and themagnetic core assembly 40. The resilient device 186 is configured to bedeformed between the biasing device 100 and the magnetic core 40 whenthe magnetic core assembly 40 has moved, by the linear vibrationalmotion, away from the equilibrium position by a predetermined non-zerothreshold amplitude.

The resilient device 186 comprises a pair of first and second flatspring elements 188, 190. Each of the first and second flat springelements 188, 190 has an outer edge 192, 194 fitted to the magnetic coreassembly 40 and a free inner edge 196, 198 spaced radially outwardlyfrom the mast 4 and spaced axially inwardly of the respective first andsecond plate spring 102, 104. The outer edge 192 of the first flatspring element 188 is fitted to the first end part 118 of the magneticcore assembly 40 and the outer edge 194 of the second flat springelement 190 is fitted to the second end part 120 of the magnetic coreassembly 40.

Typically, the outer edge 192, 194 of each of the first and second flatspring elements 188, 190 is fitted to the magnetic core assembly 40 bybeing urged by a spring so as to be securely retained in positionagainst the magnetic core assembly 40. As shown schematically in FIG. 1, a spring bias element 191 is provided between the outer edge 192, 194and respective first or second plate spring 102, 104 which urges theouter edge 192, 194 firmly against the first or second end part 118, 120of the magnetic core assembly 40. In an alternative, although lesspreferred, embodiment, the outer edge 192, 194 of each of the first andsecond flat spring elements 188, 190 may be otherwise fitted, forexample directly fixed, to the magnetic core assembly 40.

The first and second spacers 178, 180 extend radially outwardly of themast 4 and define respective first and second faces 200, 202 each ofwhich is oriented inwardly towards, and spaced from, in a directionalong the axis A-A, the respective free inner edge 196, 198 of therespective first and second flat spring element 188, 190. In theillustrated embodiment, the first and second faces 200, 202 are spacedfrom, in the direction along the axis A-A, the respective free inneredge 196, 198 of the respective first and second flat spring element188, 190 by a respective gap 204, 206 having a predetermined length.

Preferably, only the outer edge 192, 194 of each of the first and secondflat spring elements 188, 190 is in contact with any other part of theelectromechanical generator 2. Each of the first and second flat springelements 188, 190 has an inner surface 208, 210 which faces the magneticcore assembly 40, and a peripheral portion 212, 214 of each innersurface contacts the magnetic core assembly 40. The peripheral portion212, 214 of each inner surface 208, 210 contacts an upstandingperipheral edge 216, 218 of the magnetic core assembly 40.

The high degree of magnetic coupling between the movable magnetic coreassembly 40 and the coil 12, and the high mass of the movable magneticcore assembly 40, enables the resonant frequency readily to be tunedaccurately to a desired value, and also permits a high self-restoringforce to be applied to the movable magnetic core assembly 40 during itsresonant oscillation to minimize the amplitude of the oscillation. Sincethe amplitude is limited, the springs 102, 104 are only ever deformed bya very small degree, well within their linear spring characteristics.Typically, under normal operation the maximum amplitude is less thanabout 1 mm.

The springs 102, 104 bias, back towards the central equilibriumposition, the magnetic core assembly 40 which can move axially along theaxis A-A when the electromechanical generator 2 is subjected to anapplied mechanical force, in particular a mechanical vibration, havingat least a component along the axis A-A. The springs 102, 104 have ahigh stiffness in the lateral, i.e. radial, direction so assubstantially to prevent non-axial movement of the magnetic coreassembly 40.

The generator 2 is configured such that the mass of the magnetic coreassembly 40 is permitted to oscillate about the equilibrium pointrelative to the mast 4 with an oscillation amplitude no more than thepredetermined threshold amplitude without the resilient device 186,comprising the first and second flat spring elements 188, 190, beingdeformed, i.e. flexed, between the biasing device 100 and the mass.Accordingly, in such a scenario of “normal operation”, the resilientdevice 186 does not cause any power loss from the generator 2 when theoscillation amplitude of the mass is no more than the particular orpredetermined threshold amplitude.

However, the generator 2 is configured such that, when the oscillationamplitude exceeds the predetermined threshold amplitude, such as when itis subjected to a severe shock, the resilient device 186 is thendeformed, i.e. flexed, between the biasing device 100 and the mass toact as a limiter that limits the oscillation amplitude. Accordingly, theelectromechanical generator 2 according to the preferred embodiments ofthe present invention has particular utility in environments where itmay be subjected to occasional severe shocks.

The first and second flat spring elements 188, 190 respectively impactthe first and second spacers 178, 180 to provide the amplitudelimitation. The first and second spacers 178, 180 provide the advantagethat the initial gap 204, 206 between the spacers 178, 180 and the flatspring elements 188, 190 can be accurately set. Therefore the firstspacer 178 and the second spacer 180 can function as shims between therespective first and second plate springs 102, 104 and the mast 4, todefine a predetermined distance between the first and second spaces 178,180 and the first and second flat spring elements 188, 190. Also, theamplitude limiting motion of the first and second flat spring elements188, 190 against the first and second spacers 178, 180 avoids orminimizes sliding motion, which eliminates or minimizes wear. The firstand second flat spring elements 188, 190 may be made of phosphor-bronzeand the first and second spacers 178, 180 may be made from steel. Thesematerials can provide the required high spring constant to the flatspring elements 188, 190, which is preferably higher than the springconstant for the first and second plate springs 102.

The electromechanical generator 2 may be disposed with in a housing,which may be hermetically sealed to protect the mechanical andelectrical parts of the electromechanical generator 2. The interiorvolume of the housing may include an inert gas.

The electromechanical generator 2 uses a resonant mass-springarrangement. If the electromechanical generator 2 is subject to a sourceof external vibration that causes it to move along the direction A-A,then the magnetic core assembly 40 comprises an inertial mass which maymove relative to the mast 4, also along the direction A-A. In doing so,the springs 102, 104 are deformed axially, and work is done against adamper comprising the static electrical coil 12 and the movable magneticcore assembly 40 that generates a region of magnetic flux within whichthe electrical coil 12 is disposed. Movement of the electrical coil 12within the magnetic flux causes an electrical current to be induced inthe electrical coil 12 which can be used as a source of electrical powerfor driving an external device (not shown).

By increasing the electrical output, as a result of increased magneticcoupling, the operating band width of the device can be greatlyincreased. This in turn greatly enhances the ability of the device to beused in many new energy harvesting applications.

Simple plate springs 102, 104 can be employed in the electromechanicalgenerator 2. This provides a reliable and simple structure to preventlateral movement on the magnetic core assembly 40, with low friction andavoiding complicated, intricate and/or expensive manufacturingtechniques. The resultant structure is robust and compact. Since theplate springs 102, 104 are subjected to a very low amplitude ofdeformation, their mechanical properties are not especially critical,because they are never deformed anywhere near their mechanical limits oflinear elastic movement, and so they can accordingly be of relativelyconventional quality, and consequently have a low component cost.

In the electromechanical generator of the preferred embodiment of thepresent invention a high moving mass can be achieved by filling almostall of the internal space of a housing of the device with a metallicmagnetic core assembly. This can be achieved at least partly becauseflat springs at opposed ends of the magnetic core assembly are volumeefficient. In addition, a high Q comes from the fact that the “enclosed”structure of the magnetic core assembly leaks very little flux, and sothere is very little eddy current in the surrounding material of thestationary housing. Accordingly, little clearance needs to be keptbetween the moving magnetic core assembly and the housing, allowing moremoving mass. A high magnetic coupling comes also from the enclosednature of the magnetic core assembly where very little flux leaksout—almost all the magnetic flux gets channeled through the coil.

Other modifications and embodiments of the present invention as definedin the appended claims will be apparent to those skilled in the art.

The invention claimed is:
 1. An electromechanical generator forconverting mechanical vibrational energy into electrical energy, theelectromechanical generator comprising: a central mast, an electricallyconductive coil assembly fixedly mounted to the mast, the coil assemblyat least partly surrounding the mast, the coil assembly having radiallyinner and outer sides and first and second opposite edges, a mount forthe coil assembly extending radially inwardly of the radially inner sideand fixing the coil assembly to the mast, a magnetic core assemblymovably mounted to the mast for linear vibrational motion along an axisabout an equilibrium position on the axis, the magnetic core assembly atleast partly surrounding the coil assembly and the mast, wherein themagnetic core assembly comprises: an outer core, comprising a one-piecetubular body, which encloses the electrically conductive coil assemblyon the radially outer side, first and second end cores magneticallycoupled to the outer core at respective first and second ends of theouter core, the first and second end cores extending radially inwardlyand enclosing the respective first and second opposite edges of the coilassembly, wherein either (i) both of the first and second end cores arefitted to and contact the outer core at the respective first and secondends of the outer core, or (ii) one of the first and second end cores isfitted to and contacts the outer core at the respective first or secondend of the outer core and the other of the first and second end cores isintegral with the outer core at the respective first or second end ofthe outer core, wherein the first and second ends of the tubular bodycomprise a recess on an inner side of the tubular body, the first andsecond end cores are fitted in the recess of the respective first andsecond ends of the tubular body, the recess has a transverse mountingsurface facing along the axis away from the equilibrium position and alongitudinal mounting surface facing towards the axis, and radial andcircumferential surfaces of the respective first and second end coresare respectively fitted to the transverse and longitudinal mountingsurfaces; first and second magnets spaced along the axis, wherein thefirst and second magnets contact and are magnetically coupled to therespective first and second end cores, and the first and second magnetsdefine therebetween a gap in the magnetic core assembly through whichthe mount extends, and first and second locator elements respectivelyfitted to the first and second end cores, the first and second locatorelements each extending towards the mount, wherein the first and secondlocators elements are fitted to a fitting surface of the respectivefirst and second end cores, the fitting surface facing towards the axisand being an inner end surface of then respective first and second endcore, and wherein each of the first and second locator elements has alocation surface which faces away from the axis and which engages, andis fitted to, a side surface of respective first and second magnet, theside surface facing toward the axis and being an inner surface of therespective first or second magnet.
 2. An electromechanical generatoraccording to claim 1 wherein the tubular body is cylindrical.
 3. Anelectromechanical generator according to claim 1 wherein the first andsecond end cores are circular, each having an outer circumferentialsurface fitted to an inner circumferential surface of the outer core anda central hole surrounding the mast.
 4. An electromechanical generatoraccording to claim 1 wherein the first and second end cores comprisesplates.
 5. An electromechanical generator according to claim 1 whereinthe magnetic core assembly comprises two opposed magnetic circuitsspaced along the axis.
 6. An electromechanical generator according toclaim 1 wherein the poles of the first and second magnets have a firstcommon polarity facing towards each other, and poles of the first andsecond magnets facing away from each other having a second commonpolarity.
 7. An electromechanical generator according to claim 1 whereinthe first and second end cores and the outer core comprise aferromagnetic body magnetically coupled to the first and second magnets.